1. Field of the Invention
This invention relates to an apparatus for monitoring and diagnosing deterioration of a exhaust gas purifying catalyst used in an exhaust system of an automobile engine.
2. Description of Related Art
Engines, in particular automobile engines, are typically provided with an exhaust system with an exhaust gas purifying catalyst installed therein. Such an exhaust gas purifying catalyst possibly encounters a hardly negligible decline in gas purifying ratio due to thermal deterioration. In order to monitor and diagnose functional deterioration of the exhaust gas purifying catalyst, the utilization is made of a pair of oxygen (O.sub.2) sensors disposed before and after the exhaust gas purifying catalyst in the exhaust system. Functional deterioration is monitored based on outputs from the oxygen (O.sub.2) sensors. One of the catalyst deterioration monitoring techniques is known from, for example, Japanese Unexamined Patent Publication No. 6-249029.
Catalyst deterioration detection is performed by comparing a value relating to a ratio between the numbers of reversal of outputs, i.e. a ratio between reversal cycles of outputs, from the two oxygen (O.sub.2) sensors with a threshold value. For example, a ratio of the number of reversal of output from the upstream oxygen (O.sub.2) sensor to the number of reversal of output from the downstream oxygen (O.sub.2) sensor (which is hereafter referred to as an output reversal ratio) takes a significantly large value when the exhaust gas purifying catalyst is functionally normal and, however, gradually declines with deterioration, i.e. a decline in purifying ratio, of the exhaust gas purifying catalyst. Accordingly, the exhaust gas purifying catalyst can be judged deteriorated when the output reversal ratio exceeds the threshold value.
As described in Japanese Unexamined Patent Publication No. 6-42338, the detection of deterioration of the exhaust gas purifying catalyst is interrupted upon an occurrence of a fault of a fuel injector or a fault of an exhaust gas recirculation system.
In the diagnosis of deterioration of the exhaust gas purifying catalyst performed by comparing an output reversal ratio with a threshold value, it has been proved that, although the exhaust gas purifying catalyst is functionally normal, it is apt to be judged deteriorated or that, although the exhaust gas purifying catalyst has been deteriorating, it is apt to be judged functionally normal. On study of the cause of wrong detection, it was found that the amount of exhaust gases passing the exhaust gas purifying catalyst produces a great effect on diagnosis of catalyst deterioration. In particular, The diagnosis of deterioration is greatly dominated by a ratio of the maximum amount of exhaust gas (passing through the exhaust gas purifying catalyst in a range where air-to-fuel ratio feedback control is available) relative to the capacity of the exhaust gas purifying catalyst. Specifically, the exhaust gas purifying catalyst purifies exhaust gas at a rate declined more when the amount of exhaust gas is large than when it is small. For this reason, even though the degree of deterioration of the exhaust gas purifying catalyst is the same, the output reversal ratio is smaller when the amount of exhaust gas passing through the exhaust gas purifying catalyst is large than when it is small. For a moderate amount of exhaust gas, the exhaust gas purifying catalyst shows a lenient change in gas purifying ratio relative to a change in the output reversal ratio over a significantly broad range before and after a boundary between an unacceptable deteriorated state and a functionally normal state, it is definitely discriminated between these functional states at the threshold value taken as a boundary.
In a range of large amounts of exhaust gas and in a range of small amounts of exhaust gas, the exhaust gas purifying catalyst shows a significant change in gas purifying ratio relative to even a slight change in the output reversal ratio near the boundary between the unacceptable deteriorated state and the functionally normal state. Further, in the range of large amounts of exhaust gas, the range of output reversal ratios based on which the exhaust gas purifying catalyst is judged normal is significantly broad and, however, the range of output reversal ratios based on which the exhaust gas purifying catalyst is judged deteriorated is significantly narrow. For this reason, the exhaust gas purifying catalyst is apt to be judged deteriorated in the range of large amounts of exhaust gas although it is functionally normal. To the contrary, in the range of small amounts of exhaust gas, the range of output reversal ratios based on which the exhaust gas purifying catalyst is judged normal is significantly narrow and, however, the range of output reversal ratios based on which the exhaust gas purifying catalyst is judged deteriorated is significantly broad, so that the exhaust gas purifying catalyst is apt to be judged normal in the range of small amounts of exhaust gas although it is deteriorated.
In order to prevent the exhaust gas purifying catalyst from being faultily judged, while only a moderate range of amounts of exhaust gas may be utilized to perform the deterioration diagnosis, frequency with which the deterioration diagnosis is performed is reduced.
On the other hand, during the diagnosis of detrioration of the exhaust gas purifying catalyst, in order to improve the detectability of output from the oxygen (O.sub.2) sensor for easy discrimination, it is popular to increase a gain in the air-to-fuel ratio feedback control. In such a case, unless the diagnosis of detrioration is completed, the increased gain remains effective, hunting is encountered under regular operating condition